The present invention relates generally to air seals for gas turbine engines, and relates more particularly to seals having improved properties in operating conditions during which unusually large amounts of seal material is liberated and ingested into the engine.
Gas turbine engines are well known sources of power, e.g., motive power for aircraft or as power generators, and generally include compressor (typically preceded by one or more fan stages), combustor and turbine sections. As illustrated generally in FIG. 1, compressor and turbine sections (and any fan stages) each include shaft-mounted, rotating disks 1, each carrying a set of blades 2 located within a hollow housing or case 3, with intervening sets of stationary vanes 5 mounted to the case. Air seals 4, 7 are provided between the tips of the blades and the case (outer air seals), and between the free ends 6 of the vanes and the knife edges 8 of the disks (knife edge seals) to prevent air leakage between those components.
Air is ingested through an engine inlet and compressed by rotating disks and associated blades in the compressor. The compressed air is then burned with fuel in the combustor to generate high pressure and temperature gasses, which cause rotation of the turbine sections and associated fan compressor stages and are then ejected out an engine exhaust to provide thrust. The case is intended to prevent leakage of air or combustion products around the tips of the blades, i.e., between the blade tips and the case, which leakage reduces the efficiency of the engine.
Despite the design of components to minimize leakage, a substantial proportion of any leakage which does occur in a normally-operating gas turbine engine occurs between the tips of the blades and the case, and between the tips of the vanes and the disks. One manner of eliminating such leakage is to fabricate all mating parts to extremely close tolerances, which becomes increasingly expensive as tolerances are reduced. Moreover, given the temperature ranges to which the parts are subjected to before, during and after operation, and the resultant thermal expansion and contraction of the parts, such close tolerances will at times result in interference between mating parts and corresponding component wear and other damage. Accordingly, gas turbine engine designers have devoted significant effort to developing effective air seals, and particularly seals composed of abradable materials. See, e.g., U.S. Pat. Nos. 4,936,745 to Vine et al. and 5,705,231 to Nissley et al., which are assigned to the assignee of the present invention and expressly incorporated by reference herein. Such seals require a balance of several properties including abradability upon being contacted by a rotating blade tip, erosion resistance, durability, thermal expansion balanced with that of the underlying material, and relative ease and reasonable cost of manufacture. See, e.g., U.S. Pat. No. 5,536,022 to Sileo, which is also assigned to the assignee of the present invention and expressly incorporated by reference herein.
A typical compressor air seal includes the seal substrate, e.g., a metal substrate, a metal layer composed of a metal powder plasma sprayed on the substrate, and an abradable, sealing layer which is also typically plasma sprayed onto the metal layer. A typical sealing layer includes a metal matrix of aluminum and silicon with some amount of embedded polyester powder particles.
During normal operation, small amounts of seal material are removed from the seal by the cooperating part, e.g., a rotating compressor blade tip. However, during engine operation, e.g., a bird strike or a stall or other condition, significant amounts of seal material are liberated by the rotating blades and is ingested into the engine, thus increasing the concentration of metallic and filler material (such as polyester) particles in the ingested air. In addition, the blades rub against the seal with more force than normal and also generate a significant amount of heat over and above the heat due to compression of the air being compressed as it is moved through the engine (and also sometimes produces sparks). In some instances, the significantly higher concentration of polyester ingested into the engine coupled with the heat or sparks leads to deflagration in the engine, and significant damage to or destruction of the engine. While the metal particles can also ignite, the filler materials typically require less energy to ignite and thus ignite before the metal. Such damage can cost millions of dollars in engine damage alone and also poses a safety hazard to persons in proximity to the engine, e.g., passengers on a plane or workers in a power generating plant. While these conditions are not encountered frequently, the design of gas turbine engines should provide for encountering such conditions during engine testing and/or operation, and accordingly must be addressed or resulting serious engine damage or other injuries must be taken for granted.
One proposed solution to the problem has been to heat treat the seal after the materials are deposited by plasma spraying, e.g., to remove the polyester or other filler material from the seal. The polyester (or other filler) provides spacing for the metal matrix, and also absorbs some of the heat generated during rubbing of the seal and the cooperating component. For lightweight seal systems, such as aluminum or similar metal matrix, the removal of the polyester prevents deflagration due to the presence of polyester. However, the metal matrix (of Al and Si) can be damaged during filler removal, e.g., due to the expansion of the filler as it is heated and prior to vaporization and by the heat required to remove the filler. The resulting seal is structurally weak (due to significant porosity), exhibits relatively poor erosion resistance, and is susceptible to significant densification and corresponding poor abradability and sealing. Abradability and erosion resistance are important characteristics for seal materials, and accordingly the proposed solution for this seal material renders the material not appropriate for use in its intended purpose.
Another proposed solution was to apply a seal material utilizing a different filler. The material selected is sold commercially by Sulzer Metco under the designation Metco 320, and is composed of aluminum and silicon powder particles, with hexagonal boron nitride powder particles bound to the aluminum and silicon using an organic adhesive. Such material is also referred to as a "composited powder". This composited powder was delivered as a single source of material to a plasma spray gun and was applied to a seal substrate. During testing, the seal material lacked sufficient amounts of filler material and thus lacked sufficient abradability, and we determined that this condition was due to an inability to bind a sufficient amount of filler to the metal particles. Efforts to increase the amount of filler particles adhered to the metal proved unsuccessful, even after filler particles were adhered to substantially the entire surfaces of the metal particles. Moreover, the adhesive did not vaporize during the plasma spray process and was deposited with the powder materials, and merely added weight without favorably affecting either durability or abradability. During use, this seal material tended to densify and lose its abradability, thus detrimentally affecting seal performance.
It is an object of the present invention to provide a gas turbine engine air seal that provides acceptable durability and abradability and yet will not deflagrate in the event that unusually large amounts of the seal material are ingested into the engine, e.g., during off-design operation.
It is another object to provide such a seal that is also cost effective.
It is yet another object to provide a seal that weighs no more than conventional seal material, and provides no weight penalty.
It is still another object to provide such a seal using conventional equipment.
It is a still further object to provide a process of providing such a seal that enables adjustment of the proportion of metal and of filler, to provide an optimal seal adapted for different operating conditions.